Process for Producing Ti and Apparatus Therefor

ABSTRACT

A process for producing Ti, comprising a reduction step of reacting TiCl 4  with Ca in a CaCl 2 -containing molten salt having the Ca dissolved therein to thereby form Ti particles, a separation step of separating the Ti particles formed in said molten salt from said molten salt and an electrolysis step of electrolyzing the molten salt so as to increase the Ca concentration, wherein the molten salt increased in Ca concentration in the electrolysis step is introduced into a regulating cell to thereby render the Ca concentration of the molten salt constant and thereafter the molten salt is used for the reduction of TiCl 4  in the reduction step. In the present invention, the Ca concentration of the molten salt to be fed to the corresponding reduction vessel can be inhibited from fluctuating and, at the same time, can maintain high concentration levels. Further, a large volume of the molten salt can be treated continuously. Therefore, the reduction reaction of TiCl 4  can be efficiently performed, and the process can be effectively utilized in the production of Ti by Ca reduction as a production process for realizing Ti production on an industrial scale.

TECHNICAL FIELD

The present invention relates to a process for producing metallic Ti by reduction treatment of TiCl₄ with Ca and to an apparatus for use in that process.

BACKGROUND ART

A common industrial process for producing metallic Ti is the Kroll process comprising reducing TiCl₄ with Mg. In this Kroll process, metallic Ti is produced via a reduction step and a vacuum separation step. In the reduction step, liquid-form TiCl₄ fed from above into a reaction vessel is reduced by molten Mg, whereupon granular metallic Ti is formed and it then gradually settles downward to give spongy metallic Ti. In the vacuum separation step, the unreacted Mg and the byproduct MgCl₂ are removed from the spongy metallic Ti in the reaction vessel.

In the production of metallic Ti by the Kroll process, it is possible to produce high-purity products. However, since the process is a batch-wise one, the production costs increase and the prices of the products become very high. One of the causes for the increased production costs is the difficulty in increasing the rate of feeding of TiCl₄.

While several reasons therefor are conceivable, one is that when the rate of feeding of TiCl₄ is excessively increased, TiCl₄ is supplied from above onto the MgCl₂,failing to move downwards and staying on the liquid surface and, therefore, the supplied TiCl₄ is partly discharged from the reaction vessel in the form of unreacted TiCl₄ gas and/or insufficiently reduced TiCl₃ gas, among others, resulting in decreasing utilization efficiency of TiCl₄.

Further, in the Kroll process, the reactions are carried out only in the vicinity of the liquid surface of molten Mg in the reaction vessel, so that the heat-liberating area is narrow. Therefore, cooling will not be able to keep up with the supply of TiCl₄ when supplied at a high rate; this is also a major reason for the rate of feeding of TiCl₄ being restricted.

Further, due to the wettability (stickiness) of molten Mg, the Ti powders formed settle in a flocculated manner and are sintered and grown in grain size during settling by the heat which the high-temperature molten liquid has, rendering it difficult to recover those out of the reaction vessel. As a result, the production of metallic Ti cannot be carried out continuously, and the productivity is impeded.

As regards another process for producing Ti other than the Kroll process, the specification of U.S. Pat. No. 2,205,854 describes that Ca can be used as a reducing agent other than Mg for reduction of TiCl₄. And, as a process for producing Ti using the reduction reaction with Ca, a method is described in the specification of U.S. Pat. No. 4,820,339 (hereinafter referred to as “Document 1”) which comprises retaining CaCl₂ in a molten salt form in a reaction vessel, feeding metallic Ca powders into the molten salt from above and allowing the Ca powders to dissolve in the molten salt and, at the same time, supplying gaseous TiCl₄ from below for causing TiCl₄ to react with the molten Ca in the molten salt of CaCl₂.

However, the process described in Document 1 cited above cannot be put into a commercial use as the process for producing Ti since the metallic Ca powders to be used as the reducing agent are very expensive and, if they are purchased and used, the production costs will become higher as compared with the Kroll process. In addition, it is very difficult to handle Ca which is highly reactive; this is also an important factor hindering the industrial use of the Ti production process by Ca reduction.

As yet another process for producing Ti, Olson's process is described in the specification of U.S. Pat. No. 2,845,386 (hereinafter referred to as “Document 2”) which comprises directly reducing TiO₂ with Ca without going through the stage of TiCl₄. This process is a kind of direct oxide reduction method. However, the use of high-purity TiO₂, which is expensive, is inevitable in this process.

On the other hand, the present inventors considered that the reduction of TiCl₄ with Ca is essential for the establishment of an industrial process for producing Ti by Ca reduction and that it is necessary to supplement the Ca in the molten salt consumed in the reduction reaction in an economical manner, and they proposed, in Japanese Patent Application Publication No. 2005-133195 (hereinafter referred to as “Document 3”) and Japanese Patent Application Publication No. 2005-133196 (hereinafter referred to as “Document 4”), a process which utilizes the Ca formed upon electrolysis of molten CaCl₂ and recycling this Ca, namely “OYIK method”. Document 3 cited above describes a process in which Ca is formed and replenished by electrolysis and the Ca-enriched molten CaCl₂ is introduced into a reaction vessel and used for the formation of Ti particles by Ca reduction, and Document 4 cited above further discloses a method of effectively inhibiting the back reaction resulting from electrolysis through the use of an alloy electrode (e.g. Mg—Ca alloy electrode) as the cathode.

DISCLOSURE OF INVENTION

As mentioned above, a number of research and development works have so far been made concerning processes for producing Ti other than the Kroll process. In particular, in the OYIK method proposed by the present inventors, Ca in molten salt is consumed with the progress of the reduction reaction of TiCl₄ but when that molten salt is electrolyzed, Ca is formed in the molten salt; when the thus-obtained Ca is reused in the reduction reaction, Ca replenishment from outside becomes unnecessary and, furthermore, it is unnecessary to isolate Ca singly, thereby enhancing the economical efficiency.

Accordingly, the present inventors made investigations concerning all the production steps in an attempt to further develop a process for producing metallic Ti, the fundamental configuration of which is based on the OYIK method and by which the operation can be carried out in a more efficient and stable manner. The process for producing Ti or a Ti alloy according to the present invention, which is an evolutionary form of the OYIK method, is named “OYIK-II method” after the initials of the four persons “Ogasawara, Yamaguchi, Ichihashi and Kanazawa” who had been deeply involved in coming up with an idea, development and completion of that process.

It is an object of the present invention to provide a process for producing Ti and a production apparatus therefor which are to be employed in producing metallic Ti by reduction of TiCl₄ with Ca formed by electrolysis of molten CaCl₂ and which makes it possible to allow the reduction reaction of TiCl₄ to proceed efficiently and carry out the operation in a stable manner on an industrial scale.

For attaining the above object, namely, for allowing the reduction reaction of TiCl₄ to proceed efficiently and making it possible to carry out the operation in a stable manner, it is important to increase the Ca concentration in the CaCl₂-containing molten salt to be fed to a reduction vessel for reducing TiCl₄ and suppress the fluctuations in Ca concentration and, for enabling Ti production on an industrial scale, it is necessary to increase the rate of feeding of Ca to the reduction vessel (or, in other words, to enable continuous treatment of a large volume of the CaCl₂-containing molten salt in the electrolysis step).

In case where the Ca concentration in the molten salt fed to the reduction vessel is too low, the unreacted TiCl₄ gas is discharged out of the vessel. In addition, such titanium subchlorides as TiCl₃ and TiCl₂ are formed as gases and are dissolved in the molten salt and, when this molten salt is returned to an electrolyzer for electrolyzing CaCl₂ to form Ca, the Ca thus formed may react with the titanium subchlorides to form Ti, depositing on a surface of a cathode, which may possibly cause shortcircuiting between the electrodes and/or clogging within the cell, depending on the configuration of the electrolysis cell. It is also feared that TiC, which is a cause of C contamination for Ti, be formed.

On the other hand, when the Ca concentration in the molten salt is excessively high, the molten salt extracted from the reduction vessel contains a large amount of Ca, and part of the Ca-containing molten salt remains adhered to the Ti separated from the molten salt in the separation step. This residual molten salt is completely removed from the Ti, separated and recovered, in the step of melting, while Ca evaporates, causing a loss and, further, it adheres to the inside wall of the melting furnace, making it necessary to remove it by large-scale cleaning.

In addition, the Ca concentration in the molten salt after separation of Ti in the separation step is also high, so that when the molten salt is returned to the electrolyzer, the Ca reacts with chlorine formed by electrolysis (back reaction), lowering the current efficiency. Further, the temperature uniformity in the molten salt (bath salt) in the electrolyzer is disturbed by the heat of reaction generated on that occasion and the temperature control of the bath salt may be possibly disturbed.

Therefore, it is preferable that the Ca concentration in the molten salt fed to the reduction vessel does not fluctuate and always is kept constant and at a high level so as to proceed the reduction reaction efficiently. However, it is very difficult to control the Ca concentration at a constant level while measuring, for example, the Ca concentration in the molten salt extracted from the electrolyzer on a real-time basis; fluctuations in Ca concentration as being incurred from certain changes in the electrolysis conditions in the electrolyzer are unavoidable. Therefore, it is difficult to always maintain the Ca concentration at a constant level so long as the technique of feeding the molten salt, increased in Ca concentration in the electrolyzer, directly to the reduction vessel is employed.

Accordingly, the present inventors made investigations for the purpose of suppressing fluctuations in Ca concentration in the molten salt fed to the reduction vessel and maintaining the Ca concentration at a high level. As a result, they found that it is effective to provide a regulating cell comprised of a Ca supply source, between the electrolyzer (hereinafter referred to as “principal electrolyzer”) and the reduction vessel so that the molten salt increased in Ca concentration in the principal electrolyzer may be introduced into the regulating cell and, after rendering the Ca concentration thereof constant therein, the molten salt may be used in the reduction step. It was also found that a molten Ca—Mg alloy is suited for use as the Ca supply source.

The inventors further made detailed investigations concerning the principal electrolyzer container configuration, electrode configuration, electrolysis conditions and distance between electrodes, among others, and, as a result, found that when the electrolysis is carried out while the molten salt is caused to flow in one direction in the vicinity of the surface of the cathode and the molten salt increased in Ca concentration is recovered on the outlet side of the principal electrolyzer, the back reaction can be inhibited, the current efficiency can be maintained at high levels and, thus, only the molten salt enriched in Ca can be taken out effectively and, in addition, a large volume of the CaCl₂-containing molten salt can be continuously treated.

The present invention has been completed based on such findings and it consists in the process for producing Ti as defined below under (1) and the apparatus therefor as defined below under (2).

(1) A process for producing Ti, comprising a reduction step of reacting TiCl₄ with Ca in CaCl₂-containing molten salt having Ca dissolved therein to form Ti particles in the molten salt, a separation step of separating the Ti particles formed in the molten salt from the molten salt and an electrolysis step of electrolyzing the molten salt of decreased Ca concentration in association with formation of the Ti particles to thereby increase the Ca concentration, wherein the molten salt of increased Ca concentration through the use of a principal electrolyzer in the electrolysis step is introduced into a regulating cell having a Ca supply source and brought into contact with the Ca supply source to thereby render the Ca concentration of the molten salt constant and thereafter it is used for reduction of TiCl₄ in the reduction step.

The term “CaCl₂-containing molten salt” as used herein refers to molten CaCl₂ alone or molten salt obtained by adding KCl, CaF₂ and/or the like to molten CaCl₂ for lowering the melting point and adjusting the viscosity, among others. Hereinafter, such molten salt is referred to simply as “molten salt”.

In the production process according to the present invention, the Ca supply source is preferably a Ca—Mg alloy since Ca of this molten Ca—Mg alloy can be replenished with ease (hereinafter referred to as “first mode of embodiment”).

When, in this first mode of embodiment, the Ca concentration of the molten Ca—Mg alloy is increased for Ca replenishment by electrolyzing the CaCl₂-containing molten salt in an electrolyzer for use in the alloy, the Ca replenishment can be carried out with ease without exerting any influence on the operation (hereinafter referred to as “second mode of embodiment”).

When, in the production process according to the present invention, the molten salt after separation of Ti particles in the separation step is once fed to the electrolyzer for use in the alloy and, after lowering the Ca concentration in the molten salt, is fed to the principal electrolyzer, the residual Ca in the molten salt fed back from the separation step to the electrolysis step can be preferably utilized effectively (hereinafter referred to as “third mode of embodiment”).

When the regulating cell to be used in the production process according to the present invention is provided with a cooling function, the temperature rise in the reduction vessel due to the exothermal reaction in the subsequent step can be moderated and the Ca concentration of the molten salt to be fed to the reduction vessel can be always maintained at a constant and high level, hence the reduction reaction can be carried out efficiently, thereby contributing to the stable operation (hereinafter referred to as “fourth mode of embodiment”).

(2) An apparatus for producing ⁻Ti which comprises: a reduction vessel for holding a CaCl₂-containing molten salt having Ca dissolved therein and reacting the Ca with TiCl₄ fed into the molten salt to form Ti particles; separation means for separating the Ti particles formed in the molten salt from the molten salt; a principal electrolyzer which is equipped with an anode and a cathode and intended for holding the molten salt after the Ti particles therein being separated and electrolyzing the molten salt to form Ca on the cathode side; and a regulating cell having a Ca supply source in which the molten salt from the principal electrolyzer is introduced and brought into contact with the Ca supply source to thereby render the Ca concentration thereof constant, subsequently followed by feeding the molten salt thus regulated to the reduction vessel.

When, in the production apparatus according to the present invention, the Ca supply source is a molten Ca—Mg alloy and there is provided an alloy electrolyzer so as to increase the Ca concentration of the molten Ca—Mg alloy, the apparatus can be suitable for applying the process for producing Ti in the above-mentioned first mode or second mode of embodiment.

When the apparatus is configured such that the alloy electrolyzer is disposed between a high-temperature decanter for the separation step and the principal electrolyzer, and the molten salt of increased Ca concentration in the alloy electrolyzer can be introduced into the regulating cell, the apparatus is suited for applying the process for producing Ti in the above-mentioned third mode of embodiment.

In the production process according to the present invention, since the molten salt of increased Ca concentration in the principal electrolyzer is introduced into the regulating cell having a Ca supply source to thereby adjust the Ca concentration thereof to a constant level and thereafter is used for reduction of TiCl₄, the Ca concentration of the molten salt to be fed to the reduction vessel can be inhibited from fluctuating and can be maintained at a high level. Accordingly, it becomes possible to allow the reduction reaction of TiCl₄ to proceed efficiently and carry out the operation in a stable manner. Further, it is also possible to continuously treat a large volume of the CaCl₂-containing molten salt in the electrolysis step and increase the rate of feeding Ca to the reduction vessel, with the result that Ti can be produced on an industrial scale. This production process can be applied in an easy and appropriate manner using the production apparatus according to the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a configuration example of an apparatus for producing Ti according to the present invention.

FIG. 2 is a schematic representation of another configuration example of the apparatus for producing Ti according to the present invention.

FIG. 3 is an explanatory drawing illustrating how a molten Ca—Mg alloy is replenished with Ca by an alloy electrolyzer.

FIG. 4 is a schematic representation of a configuration example of the apparatus for producing Ti according to the present invention in which an alloy electrolyzer is incorporated.

FIG. 5 is a schematic representation of a configuration example of the apparatus for producing Ti according to the present invention in which an alloy electrolyzer is incorporated in a route for molten salt to return from a separation step to a principal electrolyzer.

FIG. 6 is a longitudinal sectional view illustrating a configuration example of essential parts of an electrolyzer which is used in applying a method for electrolyzing molten salt adopted in the present invention.

FIG. 7 is a schematic representation of a configuration example of a part of the electrolyzer in which a hollow cathode is used in applying a method for electrolyzing molten salt adopted in the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, the production process and production apparatus according to the present invention are described more specifically, referring to the drawings.

FIG. 1 is a schematic representation of a configuration example of an apparatus for producing Ti according to the present invention. As shown in FIG. 1, this apparatus comprises: a reduction vessel 1 for holding CaCl₂-containing molten salt with Ca dissolved therein and reacting the Ca with TiCl₄ fed into the molten salt to form Ti particles; separation means for separating the Ti particles formed in the molten salt from the molten salt; a principal electrolyzer 5 which is equipped with an anode 2 and a cathode 3 and intended for holding the after-separation molten salt, after the Ti particles therein being separated, and electrolyzing the relevant molten salt to form Ca on the cathode side; and a regulating cell 6 having a Ca supply source in which the relevant molten salt from the principal electrolyzer 5 is introduced, rendering the Ca concentration thereof constant, and then, the treated molten salt is sent to the reduction vessel 1. In this example, a diaphragm 4 is provided between the anode 2 and cathode 3.

Further, in the production apparatus shown in FIG. 1, there are used a decanter type centrifugal precipitator (high-temperature decanter) 7 and a separation vessel 8 as the separation means.

The production process according to the present invention comprises: a reduction step of reacting TiCl₄ with Ca in CaCl₂-containing molten salt having the Ca dissolved therein to form Ti particles in the molten salt; a separation step of separating the Ti particles formed in the molten salt from the molten salt; and an electrolysis step of electrolyzing the molten salt of a decreased Ca concentration in association with formation of the Ti particles to thereby increase the Ca concentration, and is characterized in that the molten salt of an increased Ca concentration through the use of a principal electrolyzer in the electrolysis step is introduced into a regulating cell having a Ca supply source and brought into contact with the Ca supply source to thereby render the Ca concentration of the molten salt constant and thereafter it is used for reduction of TiCl₄ in the reduction step.

Thus, in the “reduction step” in the production process according to the present invention, such an apparatus as shown in FIG. 1 referred to the above, for instance, is used and the molten salt with Ca dissolved therein at a constant concentration as fed from the regulating cell 6 is first held in the reduction vessel 1, and the Ca in the molten salt is reacted with TiCl₄ fed via a TiCl₄ feeding port 9 to form Ti particles in the molten salt.

In this case, the molten salt is not held in a stationary state within the reduction vessel 1 but is held therein while slowly moving downwards from the upper side of the reduction vessel 1 to the lower side thereof and, during the downwards movement of the molten salt, TiCl₄ is reduced by the Ca in the molten salt to form the Ti particles.

The Ti particles formed in the reduction step are separated from the molten salt in the “separation step”. In the separation step, the Ti particles are first separated and recovered from the molten salt by means of the high-temperature decanter 7 and then the adhering molten salt, adhering to the Ti particles, is removed in the separation vessel 8.

The decanter type centrifugal precipitator is a centrifugal separator of the type such that a suspended substance is centrifugally settled by rotating a rotary cylinder at a high speed; it enables high speed treatment, is high in dehydration performance and is used in various treatment plants or the like in chemical factories. Such a type which enables high-temperature treatment has also been developed and can be applied as the high-temperature decanter 7 in this separation step.

The Ti particles taken out of the high-temperature decanter 7 are heated and melted by plasma beams radiated from a plasma torch 10 in the separation vessel 8, and molten Ti is cast into a mold 11 to give a Ti ingot 12. Meanwhile, the adhering molten salt separated from the Ti particles may possibly be contaminated with fine Ti particles. If such adhering molten salt is returned to the electrolysis step, a trouble may possibly occur; it is therefore preferable that the adhering molten salt is returned to the reduction vessel 1, as shown in FIG. 1. In addition, Ca remains in a certain amount in the adhering molten salt, so that it is reasonable, from the viewpoint of effective utilization of Ca as well, to return the adhering molten salt to the reduction vessel 1. Since the flow rate of the adhering molten salt is very slight as compared with the flow rate of the molten salt introduced into the reduction vessel 1 after adjustment of the Ca concentration to a constant level in the regulating cell 6 and, therefore, the fluctuations in Ca concentration in the molten salt introduced from the regulating cell 6 to the reduction vessel 1 is negligible.

The molten salt of a decreased Ca concentration as separated in the high-temperature decanter 7 is returned to the “electrolysis step” and introduced into the space between the cathode 3 and the diaphragm 4 in the principal electrolyzer 5 and held there. While the configuration and action, among others, of the principal electrolyzer 5 to be used in this step will be described in detail hereafter, the molten salt, in this case as well, is not held in a stationary state within the principal electrolyzer 5 but is held therein while slowly moving downwards from the upper side of the principal electrolyzer 5 to the lower side thereof and, during the flowing downwards movement of the molten salt, it is electrolyzed and the Ca concentration in the molten salt is increased.

However, fluctuations in Ca concentration due to certain changes in electrolysis conditions in the principal electrolyzer 5 are unavoidable. Therefore, if the molten salt after electrolytic treatment in the principal electrolyzer 5 is directly fed to the reduction vessel 1, the fact that the Ca concentration is not always maintained at a constant level may result in the formation of titanium subchlorides and in decreases in current efficiency due to the back reaction, as mentioned above, with the result that the efficiency of the reduction reaction of TiCl₄ will be reduced and it will become sometimes difficult to carry out the operation in a stable manner.

Therefore, in the production process according to the present invention, the molten salt of an increased Ca concentration through the use of the principal electrolyzer 5 in the electrolysis step mentioned above is introduced into the regulating cell 6 having a Ca supply source and brought into contact with the Ca supply source to thereby render the Ca concentration of the molten salt constant and thereafter is used for reduction of TiCl₄ in the reduction step.

Usable as the Ca supply source are molten metallic Ca and molten Ca—Mg alloys. Thus, molten metallic Ca or a molten Ca—Mg alloy is allowed to float on the surface of the molten salt of an increased Ca concentration, and such Ca supply source and the molten salt are kept in contact with each other. Ca is thereby fed from the Ca supply source to the molten salt to make it possible to maintain the Ca concentration at a level close to the saturation solubility when the Ca concentration of the molten salt is lower than the saturation solubility and, when the molten salt has a Ca concentration at a level of the saturation solubility thereof and contains deposited metallic Ca mixed therein, the metallic Ca floats and separates owing to the difference in specific gravity in the regulating cell 6, whereby the Ca concentration can be maintained at a level close to the saturation solubility. Furthermore, when the temperature of the molten salt on the occasion of extraction from the regulating cell 6 is controlled so as to be constant, it becomes possible to control the Ca concentration at a constant level close to the saturation solubility at that temperature.

Therefore, by disposing the regulating cell 6 and introducing the molten salt of an increased Ca concentration in the principal electrolyzer 5 into the same, it becomes possible to feed the molten salt having a constant Ca concentration close to the saturation solubility thereof to the reduction vessel 1 and allow the reduction reaction of TiCl₄ to proceed efficiently and thus carry out the operation in a stable manner, irrespective of whether the Ca concentration of the molten salt is at the saturation solubility or lower than the same.

It is to be noted, however, that when, in the principal electrolyzer 5, the electrolysis is executed to an extent such that the Ca concentration exceeds the saturation solubility, metallic Ca precipitates out within the principal electrolyzer 5, possibly causing clogging or other troubles, as mentioned above. Therefore, in increasing the Ca concentration in the principal electrolyzer 5, it is preferable that the electrolysis is carried out while controlling the Ca concentration in such a manner that it may not exceed the saturation solubility but maybe increased to a level just below the same and the molten salt having a Ca concentration which is high but lower than the saturation solubility be introduced into the regulating cell 6 and brought into contact with the Ca supply source to attain a constant Ca concentration close to the saturation solubility.

FIG. 2 is a schematic representation of another configuration example of the production apparatus according to the present invention, which is to be used in applying the process for producing Ti, like the production apparatus shown in FIG. 1. The difference in configuration from the apparatus shown in FIG. 1 consists in that a sedimentation vessel (thickener) 13, which utilizes the gravity, is used as the separation means instead of the high-temperature decanter. Although the sedimentation vessel requires a wider installment area as compared with the use of a high-temperature decanter, it is advantageous in that the power cost is lower.

The first mode of embodiment of the production process according to the present invention is a process in which a molten Ca—Mg alloy is used as a Ca supply source. The use of a molten Ca—Mg alloy as the Ca supply source is preferable since when Ca from the molten Ca—Mg alloy is dissolved in the molten salt and it becomes necessary to replenish the Ca in said alloy, the replenishment can be achieved with ease, as mentioned below.

The second mode of embodiment of the present invention is a process in which, in the above-mentioned first mode of embodiment, the Ca concentration in the molten Ca—Mg alloy is increased by electrolyzing the CaCl₂-containing molten salt in an electrolyzer for the alloy.

FIG. 3 is an explanatory drawing illustrating how the molten Ca—Mg alloy is replenished with Ca by the alloy electrolyzer. In FIG. 3, the alloy electrolyzer 14 is separated into an anode side and a cathode side by a partition wall 15, the lower part of which is provided with an opening, so that the molten salt (molten CaCl₂) may not be prevented from communicating. An anode 2 is disposed on the anode side and, on the cathode side, a molten Ca—Mg alloy 16, which is lower in specific gravity than the molten CaCl₂, constitutes the cathode. An electrode rod 17 is inserted in the molten Ca—Mg alloy 16. On the other hand, the molten salt increased in Ca concentration in the electrolysis step is introduced into the regulating cell 6, and the molten Ca—Mg alloy 16 as the Ca supply source is held thereon.

When the molten CaCl₂ is electrolyzed using this alloy electrolyzer 14, chlorine gas is generated on the anode 2, and Ca is formed at the interface between the molten Ca—Mg alloy 16 serving as the cathode and the molten CaCl₂. Since a voltage is applied (a potential difference is given) between the molten Ca—Mg alloy 16 and the electrolyzer, the formed Ca will not be dissolved in the molten CaCl₂ but is absorbed by the molten Ca—Mg alloy 16, whereby the Ca concentration of the Ca—Mg alloy is increased.

Then, the molten Ca—Mg alloy 16 increased in Ca concentration is transferred to the upper part of the molten Ca—Mg alloy 16 in the regulating cell 6 (as indicated by “Mg/Ca” in FIG. 3) and the Ca occurring in the lower part thereof is fed to (i.e. dissolved in) the molten salt, and the molten Ca—Mg alloy 16 decreased in Ca concentration is fed back to the molten Ca—Mg alloy 16 in the alloy electrolyzer 14 (as indicated by “Mg” in FIG. 3). In the alloy electrolyzer 14, the Ca formed by electrolysis of the molten CaCl₂ is absorbed by the molten Ca—Mg alloy 16 to increase the Ca concentration thereof.

In this way, when the second mode of embodiment of the present invention is carried out, the Ca replenishment for the molten Ca—Mg alloy used as the Ca supply source can be carried out with ease without exerting any influence on a process for producing Ca.

FIG. 4 is a schematic representation of a configuration example of the production apparatus in which the alloy electrolyzer shown in FIG. 3 is incorporated for carrying out the production process according to the present invention. When the first and second mode of embodiments of the present invention is carried out using this apparatus, the Ca replenishment for the molten Ca—Mg alloy used as the Ca supply source can be carried out with ease without exerting any influence on the operation.

When, in carrying out the production process according to the present invention, the operation is carried out under such reducing conditions that the Ca concentration of the molten salt is excessively low and Ca is thoroughly consumed for reduction of TiCl₄ in the reduction vessel, TiCl₄ is discharged as the unreacted gas out of the vessel and titanium subchlorides are formed, as mentioned above. Therefore, it is preferable that the supply rates of TiCl₄ and Ca are adjusted so that slight amounts of Ca may remain. However, when the residual Ca-containing molten salt, even if the amount of Ca therein is slight, is introduced in as-is condition into the principal electrolyzer, the current efficiency in the principal electrolyzer may possibly be decreased due to the back reaction between this residual Ca and the chlorine formed by electrolysis.

The third mode of embodiment of the present invention is a process in which the alloy electrolyzer used in the second mode of embodiment is used and the molten salt after separation of the Ti particles therein in the separation step is once sent to the alloy electrolyzer, after lowering the Ca concentration of the molten salt, it is sent to the principal electrolyzer; this process can cast aside concerns of decreasing current efficiency due to the back reaction, among others.

FIG. 5 is a schematic representation of a configuration example of the production apparatus in which the alloy electrolyzer is incorporated in a route for the molten salt to return from the separation step to the principal electrolyzer in the schematic configuration example shown in FIG. 1. As shown in FIG. 5, the alloy electrolyzer 14 is disposed between the high-temperature decanter 7 used in the separation step and the principal electrolyzer 5, and the molten salt after separation and recovery of the Ti particles therein is once sent to this alloy electrolyzer 14. Since a voltage is applied between the molten Ca—Mg alloy 16 in the alloy electrolyzer 14 and the electrolyzer, the Ca remaining in the molten salt is electrophoretically absorbed by the molten Ca—Mg alloy 16 and the Ca in the molten salt is thus removed. This molten salt of lowered Ca concentration is fed to the principal electrolyzer 5.

While, in FIG. 5, there is shown an example in which the alloy electrolyzer 14 is disposed between the high-temperature decanter 7 and the principal electrolyzer 5, the alloy electrolyzer 14 may be disposed between the thickener 13 and the principal electrolyzer 5 in the apparatus shown in FIG. 2.

When this third mode⁻of embodiment is employed in carrying out the production process according to the present invention, the above-mentioned decrease in current efficiency, among others, due to the back reaction in the principal electrolyzer 5 can be prevented and, at the same time, the Ca removed from the molten salt is absorbed by the molten Ca—Mg alloy 16 and again used for reduction of TiCl₄ in the regulating cell 6, so that the residual Ca in the molten salt can be effectively utilized.

The fourth mode of embodiment of the present invention is characterized in that the regulating cell to be used in the production process according to the present invention is provided with a cooling function. When this mode of embodiment is employed, two effects in the following can be expected. One is that when the molten salt after adjustment of the Ca concentration thereof in the regulating cell 6 is subjected to the reduction reaction of TiCl₄ with Ca in the subsequent reduction step, the temperature increase inside the reduction vessel due to the heat generated as a result of this reaction can be moderated to some extent by heat removal in advance from the molten salt to be fed to the reduction vessel 1.

The other effect is that the Ca saturation solubility in the molten salt can be decreased by lowering the temperature of the molten salt in the regulating cell 6. Owing to this effect, the Ca concentration of the molten salt introduced from the principal electrolyzer 5 into the regulating cell 6 can be caused to arrive at the saturation solubility by lowering the temperature in the regulating cell 6 even when the Ca concentration of the molten salt as introduced therein is lower than the saturation solubility. Even when Ca precipitates out upon cooling, it floats up and serves as a Ca supply source.

Thus, when the molten salt supplied from the regulating cell 6 is cooled and controlled so that the temperature thereof may be maintained at a constant level, the Ca concentration of the molten salt to be fed to the reduction vessel 1 can be always maintained at a constant and high level and, thus, it becomes possible to allow the reduction reaction of TiCl₄ to proceed efficiently and carry out the operation in a stable manner.

The production apparatus according to the present invention is an apparatus to be used in carrying out the process for producing Ti mentioned above. The apparatus is characterized in that it comprises a reduction vessel for holding a CaCl₂-containing molten salt with Ca dissolved therein and reacting the Ca with TiCl₄ fed into the molten salt to form Ti particles, separation means for separating the Ti particles formed in the molten salt from the molten salt, a principal electrolyzer which is equipped with an anode and a cathode and intended for holding the molten salt after separation of the Ti particles therein and electrolyzing the molten salt to form Ca on the cathode side, and a regulating cell which is provided with a Ca supply source and intended for receiving the molten salt from the principal electrolyzer and bringing the same into contact with the Ca supply source, rendering the Ca concentration of the molten salt constant, and then, feeding that molten salt to the reduction vessel.

The apparatus configuration shown by way of example either in FIG. 1 and FIG. 2 represents one of embodiment modes of the production apparatus according to the present invention and, as mentioned above, the production process according to the present invention can appropriately be carried out using such production apparatus.

The apparatus configuration shown in FIG. 4 is another schematic configuration example of the production apparatus according to the present invention and, as mentioned above, is suited for carrying out the process for producing Ti in the above-mentioned first or second mode of embodiment in which the molten Ca—Mg alloy 16 is transferred between the regulating cell 6 and the alloy electrolyzer 14 and the molten Ca—Mg alloy used as the Ca supply source is thereby replenished with Ca.

The apparatus configuration shown in FIG. 5 is a further schematic configuration example of the production apparatus according to the present invention and is constituted such that the alloy electrolyzer 14 is disposed between the high-temperature decanter 7 used in the separation step and the principal electrolyzer 5 so that the molten salt decreased in Ca concentration in the alloy electrolyzer 14 may be fed to the principal electrolyzer 5. When this apparatus is used, the process for producing Ti can be carried out with ease in the third mode of embodiment, as mentioned above.

When the production process and production apparatus according to the present invention as described above are used, it is possible to increase the Ca concentration in the CaCl₂-containing molten salt to be fed to the reduction vessel and inhibit the fluctuations in Ca concentration and, accordingly, it is possible to proceed the reduction reaction of TiCl₄ effectively and carry out the operation in a stable manner.

Furthermore, in the production process according to the present invention, the molten salt is electrolyzed in the electrolysis step while the molten salt is allowed to slowly move downwards from the upper side of the principal electrolyzer to the lower side thereof, so that the molten salt can be continuously treated in large volume and the feeding rate of Cato the reduction vessel can thus be increased and it becomes possible to produce Ti on an industrial scale.

Now, a method for electrolyzing molten salt, which enables such an industrial-scale Ti production, and an electrolyzer for use in such a method are described in detail.

FIG. 6 is a longitudinal sectional view illustrating a configuration example of essential parts of an electrolyzer which is used in carrying out a method for electrolyzing molten salt employed in the present invention.

This electrolyzer 5 comprises an electrolyzer container 5 a having a tubular (cylindrical) shape elongated in one direction and intended for holding the CaCl₂-containing molten salt as well as a like cylindrical anode 2 and a round column-shaped cathode 3 each disposed in the container 5 a along a lengthwise direction of the electrolyzer container 5 a; one end (a bottom plate 18) in a lengthwise direction of the electrolyzer container 5 a is provided with a molten salt feeding port 20 while the other end (a top plate 19) is provided with a molten salt withdrawing port 21. The surfaces of the anode and cathode are aligned facing each other in a substantially vertical direction and, further, a diaphragm 4 is disposed between the anode 2 and cathode 3 for inhibiting the passage of the Ca formed by electrolysis of the molten salt. Further, the outer surface of the anode 2 is provided with a cooling device 22.

This electrolyzer 5 is used as the principal electrolyzer in the production process according to the present invention. The method for electrolyzing molten salt to be carried out using this electrolyzer comprises feeding molten salt containing a chloride of a metal-fog forming metal from one end of the electrolyzer to a space between the anode and cathode either continuously or intermittently to thereby provide a flow rate in one direction to the molten metal in the vicinity of the surface of the cathode and thus carrying out the electrolysis while causing the molten salt in the vicinity of the surface of the cathode to flow in one direction to enhance the metal-fog forming metal concentration in the molten salt.

The “metal-fog forming metal” mentioned above is a metal such as Ca, Li, Na or Al which has characteristics that it can be dissolved in the corresponding metal chloride (namely, Ca is soluble in CaCl₂ or Li is soluble in LiCl₂) and reduce TiCl₄. Since these metals behave in a similar manner on the occasion of reducing TiCl₄ to form Ti, the case where the metal-fog forming metal is Ca is described hereinafter as a typical example.

The term “molten salt containing a chloride of a metal-fog forming metal (Ca)” refers to molten CaCl₂ alone or molten salt resulting from addition, to molten CaCl₂, of KCl, CaF₂ and/or the like for lowering the melting point and adjusting the viscosity, among others.

According to the method for electrolyzing molten salt, the CaCl₂-containing molten salt is first fed from one end of the electrolyzer 5 to a space between the anode 2 and cathode 3 either continuously or intermittently.

Since the electrolyzer 5 has a shape elongated in one direction (in the example shown, a vertically elongated tubular (cylindrical) shape) , the continuous or intermittent feeding of the molten salt from one end of the electrolyzer 5 to a space between the anode 2 and cathode 3 makes it possible to provide a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode 3 and cause the molten salt to flow in one direction in the vicinity of the surface of the cathode. In this case, it is sufficient enough for a condition such that at least the molten salt present in the vicinity of the surface of the cathode 3 flows in one direction to be attained; the whole molten salt between the anode 2 and cathode 3 may also flow in one direction. The phrase “in the vicinity of the surface of the cathode” refers to the region neighboring the surface of the cathode where the Ca formed on the surface of the cathode exists.

Although the molten salt is generally supplied continuously, the molten salt may be fed intermittently in consideration of the subsequent step, for instance; namely, the supply of the molten salt may be temporarily suspended and then resumed again. When the molten salt supply is temporarily suspended, the flow of the molten salt in the vicinity of the surface of the cathode is also suspended. Therefore, the “flow rate” on the occasion of “providing a flow rate in one direction to the molten salt in the vicinity of the surface of the cathode” in the strict sense of the term includes also the no-flow condition in which the flow rate is 0 (zero).

The molten salt is then electrolyzed. Thus, the molten salt is electrolyzed to form Ca on the surface of the cathode while it is caused to flow in one direction in the vicinity of the surface of the cathode. Since the electrolyzer 5 has a shape elongated in one direction and, further, in the example shown in FIG. 6, the distance between the anode 2 and cathode 3 is relatively short to reduce the electrolytic voltage to low, the Ca-enriched molten salt alone can be extracted effectively while the molten salt near the molten salt feeding port 20, which is low in Ca concentration, is prevented from mixing with the molten salt near the molten salt withdrawing port 21, which has an increased Ca concentration as a result of electrolysis.

The technology described in Document 2 cited above uses Ca as the reducing agent but is a direct reduction method for directly reducing TiO₂, not TiCl₄, with Ca to obtain Ti and thus is different from the method for electrolyzing molten salt to be employed in the present invention. Furthermore, in the direct reduction method described in Document 2 cited above, the carbon electrode being used as the anode is consumed as CO₂ and, in addition, titanium carbide (TiC) is formed in the molten salt, so that the Ti thus obtained is adulterated with C-contaminated Ti and the workability is deteriorated; problems may be encountered in using such Ti for a rolled metal.

Further, Document 2 cited above describes a technology of “forming a flow of molten salt in the vicinity of the cathode, in forming Ti by Ca reduction in molten salt”. However, there is no description suggesting the technological philosophy or else that the anode and cathode should be disposed facing each other along a lengthwise direction of the electrolyzer and the molten salt should be caused to flow in one direction along the surface of the cathode in the vicinity of the surface of the cathode or otherwise in a cathode space formed between the surface of the cathode and a diaphragm where a diaphragm or the like is provided, and the electrolysis should be carried out in such a condition to recover the molten salt of increased Ca concentration on the outlet side of the electrolyzer.

Therefore, the method for electrolyzing molten salt to be employed in the present invention and the technology described in Document 2 are quite different from each other even if they are common in that the molten salt is caused to form a flow in one direction in the electrolyzer.

When, in applying this method for electrolyzing molten salt, an electrolyzer in which the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and a diaphragm or a partition wall configured so that part of the molten salt may communicate therethrough is provided between the anode and cathode is used, the chlorine gas generated on the anode side can be recovered with ease. Further, preferably, the back reaction, namely the reaction between the Ca and chlorine formed by electrolysis to again form CaCl₂, can be inhibited. The term “substantially” in the above-mentioned phrase “in a substantially vertical direction” means “almost” or “approximately”, and the “substantially vertical direction” indicates the vertical direction or a direction slightly slanted from. the vertical direction.

The method for electrolyzing molten salt using such an electrolyzer comprising both the electrodes disposed facing each other in a substantially vertical direction can be properly applied using the electrolyzer shown by way of example in FIG. 6. While the system employed in the electrolyzer shown in FIG. 6 comprises feeding CaCl₂ into the electrolyzer 5 from the lower side of the electrolyzer 5 and withdrawing the same from the upper side thereof, it is also possible to employ the system comprising supplying CaCl₂ conversely from the upper side of the electrolyzer 5 and withdrawing the same from the lower side thereof.

In the electrolyzer used in the method for electrolyzing molten salt, the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and, on the other hand, the molten salt in the vicinity of the surface of the cathode is given a flow rate in one direction and the direction of the flow of the molten salt is vertical, hence the chlorine gas generated on the anode side can easily float up to the surface and can be recovered with ease.

Usable as the diaphragm to be disposed between the anode and cathode is, for example, a porous ceramic body comprising yttria (Y₂ 0 ₃). A porous ceramic body prepared by firing yttria has selective permeability such that Ca and chloride ions can permeate therethrough but metallic Ca cannot and, further, has good calcium reduction resistance such that it cannot be reduced even with Ca strong in reducing power and, therefore, is suited for use as the diaphragm in practicing the method for electrolyzing molten salt to be employed in the present invention.

When an electrolyzer provided with such a diaphragm between the anode and cathode is used, the back reaction, namely the immediate reaction of Ca formed on the cathode side with chlorine formed on the anode (graphite) side to again form CaCl₂, hardly occurs and the electrolysis can be carried out with high current efficiency.

A partition wall configured so that part of the molten salt can communicate therethrough may be used instead of the diaphragm. The partition wall does not allow the passage of metallic Ca as well as molten salt constituents such as Ca and chlorine ions, but, partially providing the partition wall with slits or holes through which the molten salt can pass enables the electrolysis to proceed and, on the other hand, restricts the passage of metallic Ca to a certain extent, thereby making it possible to suppress the back reaction.

When, in applying this method for electrolyzing molten salt, use is made of an electrolyzer having a cathode which is hollow and has gaps or holes through which the molten salt can flow from the surface of the cathode into the inside thereof so that the Ca-enriched molten salt flown into the inside of the cathode can be withdrawn out of the electrolyzer, the back reaction can be inhibited effectively.

FIG. 7 is a schematic representation of a configuration example of part of an electrolyzer in which a hollow cathode is used. As shown in the electrolyzer in FIG. 7, an anode 2 and a hollow cathode 3 a are disposed facing each other in a substantially vertical direction along a lengthwise direction of the inside of the electrolyzer 5 and a diaphragm 4 is provided between the anode 2 and cathode 3 a. The cathode 3 a is provided with gaps or holes (not shown) through which the molten salt can flow from the surface of the cathode into the inside thereof.

When the electrolyzer thus configured is used and the molten salt is withdrawn from the upper side of the hollow section of the cathode 3 a, a flow of the molten salt from the outer surface of the cathode to the inside thereof (hollow section) is formed, as shown by outlined arrows, and the Ca formed on the outer surface of the cathode 3 a is directly taken into the inside of the cathode 3 a without dispersion or migration to the anode side. As a result, the back reaction can be effectively prevented. The electrolyzer shown by way of example in FIG. 7 comprises a diaphragm 4, so that the back reaction preventing effect is much stronger as compared with the case where there is no diaphragm.

The size and positions, among others, of the gaps or holes to be provided in the hollow cathode are not particularly limited. They may properly be selected so that an effective molten salt flow toward the inner surface of the cathode may be formed, taking into consideration the distance between the surface of the anode (the diaphragm surface when a diaphragm is provided) and the outer surface of the cathode, the amount of the molten salt withdrawn (amount of the molten salt supplied) and other factors.

When, in applying this method for electrolyzing molten salt, the Ca concentration in the molten salt in the electrolyzer is controlled so that it may be at a level lower than the saturation solubility, it is possible to increase the Ca concentration to thereby increase the formation rate of TiCl₄ and, at the same time, prevent such troubles as clogging in the inside of the electrolyzer. By saying “the Ca concentration is controlled so that it may be at a level lower than the saturation solubility” above, it is meant that the electrolysis is carried out “under conditions such that the Ca concentration should be close to the saturation solubility but should not allow Ca to precipitate out”.

More specifically, the optimum electrolysis conditions, the amount of the molten salt to be withdrawn per unit time and other factors are determined empirically according to the shape of the electrolyzer container, the shapes of the electrodes, the distance between the electrodes and other factors so that the “conditions that the Ca concentration should be close to the saturation solubility but should not allow Ca to precipitate out” may be satisfied at any site showing the maximum Ca concentration in the electrolyzer. In particular, when a diaphragm or partition wall is used between the anode and cathode, the Ca concentration becomes maximum near the molten salt withdrawing port on the cathode side. Therefore, by controlling the Ca concentration at such a site at a level lower than the saturation solubility, the electrolytic operation may be carried out without allowing metallic Ca to precipitate out at any site in the electrolyzer.

In the production process according to the present invention, the molten salt increased in Ca concentration in the electrolysis step is introduced into the regulating cell having a Ca supply source and brought into contact with the Ca supply source to thereby adjust the Ca concentration of the molten salt to a high and constant level so that the above “conditions that the Ca concentration should be close to the saturation solubility but should not allow Ca to precipitate out” may be satisfied. However, it is also possible to control the Ca concentration to a certain extent by empirically selecting the optimum electrolysis conditions and the amount of the molten salt to be withdrawn, as mentioned above.

In applying the method for electrolyzing molten salt adopted in the present invention, heat of reaction is generated in large quantities in the electrolyzer and therefore it is preferable that the heat is removed effectively. More specifically, either in case where the hollow cathode mentioned above is used or in case where such is not used, it is preferable that a cooling device is disposed in the central part of the cathode to remove heat of reaction from the inside of the cathode. A tubular heat exchanger, for instance, is suited for use as the cooling device.

When a cooling device (heat exchanger) is disposed on the anode side as well, the heat removal efficiency is further enhanced. The cooling device 22 disposed so as to surround the anode 2 as shown in FIG. 6 is an example.

For increasing the yield of Ca by increasing the current supply in electrolysis, it is necessary to enlarge the surface area for current supply. It is preferable that the interior of the anode 2, namely the surface opposing the surface of the cathode in the electrolyzer shown by way of example in FIG. 6, is provided with fine concavo-convex irregularities to secure a large surface area for current supply. Applicable as the means therefor is, for example, grooving for forming grooves on the electrode surface.

In accordance with the above-mentioned method for electrolyzing molten salt, the molten salt enriched in Ca to a level close to the saturation solubility can be obtained in a relatively stable manner and metallic Ti can be produced with high efficiency while averting such troubles as clogging in the inside of the electrolyzer. Further, since the electrolysis is carried out while the molten salt is caused to flow in one direction in the vicinity of the surface of the cathode, a large volume of the molten salt can be treated continuously.

The electrolyzer to be used in applying this method for electrolyzing molten salt comprises an electrolyzer container elongated in one direction and intended for holding a CaCl₂-containing molten salt, and an anode and a cathode disposed along a lengthwise direction of the electrolyzer container, and is provided with a molten salt feeding port at one end of the lengthwise direction of the electrolyzer container for supplying the molten salt to a space between the anode and cathode and with a molten salt withdrawing port at the other end thereof for withdrawing the molten salt of increased Ca concentration as formed by electrolysis of the molten salt.

The electrolyzer shown by way of example in FIG. 6 is one mode of embodiment thereof, where the surfaces of the anode and cathode are disposed facing each other in a substantially vertical direction and a diaphragm is disposed between the anode and cathode. A partition wall configured so that part of the molten salt may communicate therethrough may be disposed therein instead of the diaphragm.

When the electrolyzer shown in FIG. 6 is used, the method for electrolyzing molten salt to be employed in the present invention can be properly applied, as mentioned above.

INDUSTRIAL APPLICABILITY

In accordance with the production process according to the present invention, the molten salt increased in Ca concentration in the electrolysis step is introduced into the regulating cell having a Ca supply source to adjust the Ca concentration to a constant level and thereafter is used for reduction of TiCl₄, so that the Ca concentration of the molten salt to be fed to the reduction vessel can be inhibited from fluctuating and can be maintained at a high level. Further, since, in the electrolysis step, the molten salt is electrolyzed while causing the same to flow in one direction in the vicinity of the surface of the cathode, it becomes possible to treat a large volume of the molten salt continuously. As a result, it is possible to allow the reduction reaction of TiCl₄ to proceed efficiently and carry out the operation in a stable manner and, further, produce Ti on an industrial scale. Therefore, the production process according to the present invention and the production apparatus according to the present invention which makes it possible to carry out the process in an easy and appropriate manner can be effectively utilized in the production of Ti by Ca reduction. 

1. A process for producing Ti which comprises the steps of: forming a flow of a CaCl₂-containing molten salt having Ca dissolved therein within a reduction vessel and feeding TiCl₄ into the flow of the molten salt to thereby react the TiCl₄ with the Ca in the molten salt to form Ti particles in said molten salt; separating the Ti particles formed in said molten salt from said molten salt; and electrolyzing the molten salt decreased in Ca concentration in association with formation of the Ti particles to thereby increase the Ca concentration thereof, wherein the molten salt increased in Ca concentration through the use of a principal electrolyzer in said electrolysis step is introduced into a regulating cell having a Ca supply source and brought into contact with the relevant Ca supply source, thereby adjusting the Ca concentration of said molten salt to a constant level, and then, it is used for the reduction of TiCl₄ in the reduction step.
 2. The process for producing Ti as claimed in claim 1, wherein said Ca supply source is a molten Ca—Mg alloy.
 3. The process for producing Ti as claimed in claim 2, wherein the Ca concentration in said molten Ca—Mg alloy is increased by electrolyzing the CaCl₂-containing molten salt in an alloy electrolyzer.
 4. The process for producing Ti as claimed in claim 3, wherein the molten salt after separation of Ti particles therein in the separation step is once fed to the alloy electrolyzer, and, after decreasing the Ca concentration in the molten salt, it is fed to the principal electrolyzer.
 5. The process for producing Ti as claimed in claim 1, wherein the regulating cell is provided with a cooling function.
 6. An apparatus for producing Ti which comprises: a reduction vessel for holding a CaCl₂-containing molten salt having Ca dissolved therein while forming a flow of the molten salt within the vessel, and for feeding TiCl₄ into the flow of the molten salt to thereby react the Ca with the TiCl₄ fed to form Ti particles, separation means for separating the Ti particles formed in said molten salt from the molten salt, a principal electrolyzer which is equipped with an anode and a cathode and intended for holding the molten salt after separation of said Ti particles and electrolyzing the relevant molten salt to form Ca on the cathode side, and a regulating cell which is provided with a Ca supply source and intended for receiving the molten salt from said principal electrolyzer and causing the same to come into contact with the Ca supply source, thereby adjusting the Ca concentration of the molten salt to a constant level, and, feeding such molten salt to said reduction vessel.
 7. The apparatus for producing Ti as claimed in claim 6, wherein the Ca supply source is a molten Ca—Mg alloy.
 8. The apparatus for producing Ti as claimed in claim 7, which is provided with an alloy electrolyzer for increasing the Ca concentration of said molten Ca—Mg alloy.
 9. The apparatus for producing Ti as claimed in claim 6, wherein said regulating cell is provided with a cooling function. 